Hewett matriculated at Lehigh University in September 1898. Following graduation in 1902, he spent another year at Lehigh as an assistant in metallurgy and mineralogy under the direction of Joseph Barrell. After Joseph Barrell moved to the Department of Geology at Yale in 1907, Hewett went there in 1909 to study geology and received his Ph.D.

In 1911, he joined the U.S. Geological Survey and his career with the organization spanned 60 years until his death in 1971. When in 1951, Donnel Foster Hewett reached the mandatory retirement age of 70, his full-time employment by the Survey was continued indefinitely by Presidential order. To a great host of geologists, he became a legend in his own time and was affectionately referred to as "Mr. Geological Survey" or "Mr. Manganese", the latter because of his devotion to and advancement of our understanding of the mineralogy and genesis of manganese ores.

During his career he received many honors:

Vice President, Geological Society of America in 1935 and 1945

President of the Society of Economic Geologists in 1936

Elected to the National Academy of Sciences in 1937 and the Academy of Arts and Sciences in 1949